In certain systems that make use of planar devices having internal conduits for conveying pressurized fluids therein, smaller dimensions will generally result in improved performance characteristics and at the same time result in reduced production and analysis costs. In this regard, miniaturized planar devices provide better performance and a more compact instrument. For example, in the application of such planar devices to instruments for performing a chromatographic analysis, such miniaturized planar devices enable faster analysis, decreased sample and solvent consumption, and increased detection efficiency.
Several approaches for the miniaturization of fluid bearing conduits in miniaturized planar assemblies have developed in the art. The conventional approach provides etched planar devices on glass, silica, metal, or ceramic substrates of moderately small size. For example, planar devices may be etched in a wafer that receives a superimposed cover plate. In some approaches, certain fluid-handling functions have not been integrated in the planar device and accordingly must be effected by use of conventional devices, such as fused silica capillary tubing, that are attached to the planar device. More recent approaches have used micromachining of silicon substrates and laser ablation of organic nonmetallic substrates to provide structures of much smaller size (i.e., microstructures) on the substrate.
Micromachining techniques applied to silicon utilize a number of established techniques developed by the microelectronics industry involving micromachining of planar materials, such as silicon. Micromachining silicon substrates to form miniaturized separation systems generally involves a combination of film deposition, photolithography, etching and bonding techniques to fabricate three-dimensional microstructures. Silicon provides a useful substrate in this regard since it exhibits high strength and hardness characteristics and can be micromachined to provide structures having dimensions in the order of a few micrometers. Examples of the use of micromachining techniques to produce miniaturized separation devices on silicon or borosilicate glass chips can be found in U.S. Pat. No. 5,194,133 to Clark et al.; U.S. Pat. No. 5,132,012 to Miura et al.; in U.S. Pat. No. 4,908,112 to Pace; and in U.S. Pat. No. 4,891,120 to Sethi et al.; Fan et al., Anal. Chem. 66(1):177-184 (1994); Manz et al., Adv. Chrom. 33:1-66 (1993); Harrison et al., Sens. Actuators, B10 (2): 107-116 (1993); Manz et al., Trends Anal. Chem. 10 (5): 144-149 (1991); and Manz et al., Sensors and Actuators B (Chemical) B1 (1-6): 249-255 (1990).
A drawback in the silicon micromachining approach to miniaturization involves the chemical activity and chemical instability of silicon dioxide (SiO.sub.2) substrates, such as silica, quartz or glass, which are commonly used in systems for both capillary electrophoresis (CE) and chromatographic analysis systems. Accordingly, Kaltenbach et al., in commonly-assigned U.S. Pat. No. 5,500,071, and Swedberg et al., in commonly-assigned U.S. Pat. No. 5,571,410 disclose a miniaturized total analysis system comprising a miniaturized planar column device for use in a liquid phase analysis system. The miniaturized column device is provided in a substantially planar substrate, wherein the substrate is comprised of a material selected to avoid the inherent chemical activity and pH instability encountered with silicon and prior silicon dioxide-based device substrates. More specifically, a miniaturized planar column device is provided by ablating component microstructures in a substrate using laser radiation. The miniaturized column device is described as being formed by providing two substantially planar halves having microstructures thereon, which, when the two halves are folded upon each other, define a sample processing compartment featuring enhanced symmetry and axial alignment.
Although the foregoing techniques are useful in the fabrication of miniaturized planar devices for effecting fluid-handling functions, there are significant disadvantages to the prior art approaches.
As shown in FIG. 1, a prior art planar device 70 may be constructed to include first and second planar substrates 71, 72 each of which include spaced channels 73, 74 that are etched or otherwise formed in respective surfaces 75, 76 by conventional techniques. Superimposition and appropriate bonding of the surfaces 75,76 may succeed in adequate alignment of the channels 73, 74 such that respective fluid-handling conduits 81-83 are created. However, deficiencies in many of the conventional techniques for forming the channels 73, 74 can result in edge effects and other asperities that create undesirable defects 78 in the channels 81-83. These defects 78 retard fluid flow and create localized reservoirs of fluid; accordingly, the defects 78 degrade the efficiency and uniformity of fluid flowing in the conduits 81-83; for example, the defects 78 can degrade the separation efficiency of a conduit that is used to construct a separation column.
Another significant problem arises in the attempt to effect hermetic sealing of the superimposed surfaces 75, 76. This step is generally carried out using adhesives which may not fully isolate the conduits 81-83, thus resulting in cross-conduit leakage. Conventional surface bonds may be prone to failure, leakage, or to degradation induced by adverse conditions, such as high temperature environments, or by the destructive nature of certain gases or liquids that may be present in the conduits 81-83.
Further, silicon substrates, and most ablatable materials such as polyimides, do not offer a sufficient combination of thermal and mechanical characteristics for the substrate to be used in certain applications. For instance: silicon materials are not ductile and cannot be folded, shaped, etc.; ablatable materials exhibit a low coefficient of thermal conductivity and are not susceptible to rapid and uniform heating or cooling, nor do they offer sufficient strength or ductility such that an ablatable substrate may be configured as a connecting member, housing, or support for other components. Furthermore, ablatable materials are expressly selected for their propensity to ablate upon the application of heat, and thus are not considered to be robust and impervious to adverse (e.g., high-temperature) environments when compared to metals and metal alloys.